1. Field of the Invention
The present invention relates to a giant magnetoresistive element used for a hard disk device, a magnetic sensor, and the like, and a method of manufacturing the same.
2. Description of the Related Art
In a giant magnetoresistive element (GMR) used for a hard disk device, a magnetic sensor, and the like, an improvement in output sensitivity and narrowing of a track have recently been advanced with increases in the recording density.
In order to improve the output sensitivity, a magnetic moment (areal moment) per unit area of a free magnetic layer is conventionally decreased by thinning the free magnetic layer to facilitate magnetization rotation of the magnetic moment. However, with the thin free magnetic layer, Barkhausen noise, thermal fluctuation noise, and the like are increased to cause the problem that a SN ratio cannot be increased even by increasing the output sensitivity. Also, a hard bias system using a permanent magnet film is conventionally used for the free magnetic layer. However, in the hard bias system, magnetization is strongly fixed at both side ends of the free magnetic layer adjacent to the permanent magnet film to produce dead zones, thereby causing the probability that the entire track region becomes a dead zone when narrowing of the track is advanced. Therefore, it is predicted that the hard bias system using the permanent magnet film is difficult to comply with a higher recording density.
Therefore, an exchange bias system has recently been proposed as the bias system for the free magnetic layer. As is generally known, a GMR element has a structure in which a first antiferromagnetic layer, a pinned magnetic layer, a nonmagnetic material layer and a free magnetic layer are laminated in turn. In the use of the exchange bias system, second antiferromagnetic layers and electrode layers are further formed on both sides of the free magnetic layer so that the track width of the GMR element is controlled by the distance between the second antiferromagnetic layers in the track width direction. In the use of the exchange bias system, no dead zone occurs, and thus output sensitivity can be possibly secured even with advances in track narrowing.
However, in the free magnetic layer, an exchange interaction acts between adjacent spins to orient the adjacent spins in parallel directions, and thus a distance corresponding to the strength of the exchange interaction between the adjacent spins is required for rotating the spins by an angle according to the strength of an external magnetic field. The strength of the exchange interaction can be represented by an exchange stiffness constant (exchange interaction constant). As the exchange stiffness constant increases, a spin direction cannot be rapidly changed to increase a distance required for spin rotation. When the distance required for spin rotation is increased, magnetization fixing at the ends of the track width region is strongly transmitted to the central portion, thereby decreasing the output sensitivity. This tendency becomes remarkable as the track width dimension decreases, and thus the output sensitivity cannot be easily secured even by using the exchange bias system. As a possible countermeasure against this, the free magnetic layer is made of a material having a small exchange stiffness constant. However, the use of the material having a small exchange stiffness constant undesirably decreases the Curie temperature. Also, the selection of the material has a limitation.
Furthermore, the use of the exchange bias system has the following problem.
Since a sensing current flows in the free magnetic layer through the antiferromagnetic layers having extremely higher resistivity than that of the electrode layers, the element resistance is increased. When the element resistance is increased, impedance is also increased to easily produce high-frequency noise, thereby failing to increase the SN ratio even with the improved output sensitivity.